Ultrasonic imaging methods using the back-propagation technique as described above are known in the art. These methods, unlike beamformer focusing of transmit beams of transducer components on predetermined lines or points by selective activation with predetermined different delays of the individual transducers, allows to activate transducers simultaneously or with such delays as to generate an unfocused or partly focused beam. Unfocused beams may be, for instance, planar acoustic waves when transmitting transducers are aligned and form a plane, such as linear planar probes or in two-dimensional planar probes. When transducers are arranged along an arched surface, their array generates a spherical or curved wave. In most cases, transmitting transducers transmit parallel ultrasonic pulses with no delay being imparted thereto. Two-dimensional arrays, in which transducers are arranged in two dimensions may themselves generate planar or arched waveforms.
While transducers are usually activated simultaneously, delays may be imparted thereto, both for having the possibility of using focusing techniques which increase the versatility of the apparatus, and for correcting wave front aberrations caused, for instance, by an imperfect alignment of ultrasound transmission sources of transducers which would cause generation of an unfocused beam with irregular fronts, and affect the resulting quality.
As an equivalence exists between the signal propagation time and the signal penetration depth within a body under examination, an equivalence also exists between the signals backscattered from the body under examination in the two forms, i.e. in the time domain and in the frequency domain. These two forms may be obtained by using Fourier transforms.
The backscattered signal received by the receiving transducers is a time-domain signal, which may be transformed into a frequency-domain signal. This transformation, substantially corresponding to a spectrum analysis, allows to use the so-called back propagation processing method, which uses, instead of time, the distance of a predetermined propagation plane from a reference plane, the latter corresponding to the plane where the received signals are detected by receiving transducers. Actually, this technique allows to calculate, from propagation depths within the body under examination, the structure of the backscattered signal by translating the reference plane in the direction of propagation in the body under examination. In this way, structural information may be obtained about scattering elements in the region under examination in any propagation plane.
Inverse transformation of the signal or signals so reconstructed from the frequency domain back into the time domain provides the data for generating an ultrasonic image. As is apparent from U.S. Pat. No. 5,628,320 and U.S. Pat. No. 5,720,708, the back propagation method allows the generation of a complete image of the transmit beam relevant region without requiring any focusing along multiple adjacent scan lines both during transmission and reception. With this method, the complete image is generated for each transmit pulse, unlike the method in which transmit beams are successively focused along individual scan lines and release a transmit pulse for each line.
Therefore, as a rule, back propagation provides a frame rate increase.
The above-mentioned U.S. Pat. No. 5,628,320 and U.S. Pat. No. 7,720,708 provide a detailed and in-depth description of the back propagation theory and method, and the information contained therein is intended as a part hereof.
It shall be noted that back propagation does not strictly require transformation of received signals from the time domain into the frequency domain before back propagation calculation, and the inverse transformation of the signals obtained by said calculation from the frequency domain into the time domain. Nevertheless, these transformation steps provide advantages that are better shown in the two above mentioned documents.
Therefore, the back propagation technique obtains very high frame rates which provide practical advantages in very few cases.
However, in most cases, conventional transmit or receive focusing on lines or points is used as it provides sufficient frame rates for imaging purposes.
Both in back propagation ultrasonic imaging and in focused beam ultrasonic imaging, typical probes have 64, 128, or 256 transducers. Ultrasonic imaging machines must have a processing channel for each transducer or a processing channel for a partial number of the receiving transducers; therefore it always has a considerable number of channels. This involves a considerable hardware complexity which is associated with high costs. For instance, each transducer must be equipped not only with ultrasonic beam forming devices, but also with a dedicated analog-to-digital converter, filters and other circuitry required for extraction of relevant information for image reconstruction. The same requirements apply for the probes whose transducer array covers only a portion of a whole scan plane and is mounted in such a manner as to be movable in the probe structure to allow scanning of a whole slice plane.
In prior art, the back propagation method does not fully obviate the hardware high cost drawback, as the only cost reduction provided thereby is limited to the removal of ultrasonic pulse focusing units. The need still exists of providing a number of processing channels that is at least equal to a submultiple of the total number of receiving transducers, but may be also equal to the number of receiving transducers itself. Typically, prior art probes have 64 to 256 electroacoustic transducers, and as a rule apparatuses have at least 64 processing channels, which is a considerable number of channels.
U.S. Pat. No. 5,720,708 discloses a typical back-propagation imaging method and apparatus for achieving a high frame rate and in which only one ultrasound pulse is transmitted into the region to be imaged, while the echo signals received by the transducer array are elaborated at the same time for constructing in one single step the entire image data. This means that the method and the apparatus have a number of channels corresponding substantially to the number of transducer in the transducer array which are activated for receiving the echo signals.
Furthermore from U.S. Pat. No. 5,477,859 a method and a device are known allowing to reduce the number of channels needed to elaborate the signals of the transducers of a transducer array for constructing an image from the said received echo signals. In order to reduce the number of channels in the signal elaboration chain U.S. Pat. No. 5,477,859 suggests to carry out an analogical signal preprocessing consisting in applying a Narrow-Band Fourier beamforming on the signals provided by groups of transducers being positioned in closed spatial relationships one relatively to the other in the transducer array. This pre-processing step furnishes for each group of transducers in a vicinity relationship a single signal which can be than fed to a dedicated channel for elaborating out from this signal the corresponding image data. Thus the number of channels necessary to the construction of an image out of the echo signals is reduced. This kind of method and the corresponding device requires an analogical preprocessing step and thus for each group of transducers an analogical pre-processing chain. Thus the signals from the transducers are submitted to an analogical process before the following digital process steps. Furthermore the costs of the analogical preprocessing units for each of the groups of transducer elements at least partly reduces the advantages of the reduction in the number of channels.